Sky Talk June 2008: Stars Twinkle – Planets Don’t

Most of us have heard the expression that “stars twinkle — planets don’t” but have no idea if it’s really true and, if so, why. With several of the brighter planets and stars currently visible in the evening sky, this is a perfect time to demonstrate its validity for yourself. And as a plus, those who are near-sighted actually are at an advantage here!

We’ve all seen stars sparkling gloriously on a crystal-clear night, especially when rising or setting. And since the planets and brighter stars look a lot alike in the night sky, it might be thought that the former should twinkle too. But they rarely do. The twinkling we see is called “atmospheric scintillation” and is caused by different layers of the air mass above our heads at night moving at various speeds and having different temperatures.

Stars, despite their often huge actual physical sizes, are so very far away that they are essentially point sources of light — a single ray streaming through space at us. As a result, any disturbance of the incoming beam by the atmosphere jiggles the star image noticeably. Planets on the other hand, while vastly smaller than most stars, are also much closer to us and display a minute disk. The end result is that planets have bundles of light rays coming at us instead of single ones like the stars. The individual atmospheric motions of the rays tend to cancel out, causing these worlds to shine with a steady light.

If you’ll set your Edmund Scientifics’ Star and Planet Locator to, say, 10 p.m. local time in mid-June you’ll notice a number of very bright stars scattered around the sky. We turn our attention to three of them in particular by facing south. Orange Arcturus lies halfway up the sky and below to the southwest is bluish-white Spica. Even lower, above the southern horizon, is fiery-red Antares. Depending on sky conditions on a given night, you’ll typically see that Arcturus twinkles slightly, Spica much more obviously so, and that Antares flickers and flashes dramatically. These differences arise because the lower a star is in the sky, the more of the atmosphere we are looking through to see it and thus the more it appears to twinkle. To intensify this effect, if you wear glasses, remove them; if you don’t wear glasses, try looking at these stars with your slightly defocused Edmund binoculars. In either case, you’ll see flashing disks of light rather than just pinpoints! Now look above the western horizon to find golden-yellow Saturn and above the eastern horizon, just rising, brilliant white Jupiter. Neither is twinkling!

There are, of course, always exceptions to rules. On hot hazy summer nights with no wind blowing, the atmosphere is often so steady and stagnant that nothing in the sky twinkles — not even stars themselves. And on some frigid crystal-clear, blustery winter nights the atmosphere is so turbulent and unsettled that everything twinkles — yes, even the planets!

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.